Electrostatic toner polymer particles are commonly prepared by a process frequently referred to as "limited coalescence". In this process, polymer particles having a narrow size distribution are obtained by forming a solution of a polymer in a solvent that is immiscible with water, dispersing the solution so formed in an aqueous medium containing a solid colloidal stabilizer and removing the solvent by evaporation. The resultant particles are then isolated, washed and dried.
In the practice of this technique, toner particles are prepared from any type of polymer that is soluble in a solvent that is immiscible with water. Thus, the size and size distribution of the resulting particles can be predetermined and controlled by the relative quantities of the particular polymer employed, the solvent, the quantity and size of the water insoluble solid particulate suspension stabilizer, typically silica or latex, and the size to which the solvent-polymer droplets are reduced by agitation.
Limited coalescence techniques of this type have been describe in numerous patents pertaining to the preparation of electrostatic toner particles because such techniques typically result in the formation of toner particles having a substantially uniform size distribution. Representative limited coalescence process employed in toner preparation are described in Nair et al. U.S. Pat. Nos. 4,833,060 and 4,965,131.
U.S. Pat. No. 5,283,151 to Santilli is representative of the prior art in this field and describes the use of carnauba wax to achieve similar toner morphology. The '151 method comprises the steps of dissolving carnauba wax in ethyl acetate heated to a temperature of at least 75.degree. C. and cooling the solution, so resulting in the precipitation of the wax in the form of very fine needles a few microns in length; recovering the wax needles and mixing therewith a polymer material, a solvent and optionally, a pigment and a charge control agent to form an organic phase; dispersing the organic phase in an aqueous phase comprising a particulate stabilizer; homogenizing the mixture; evaporating the solvent; and washing and drying the resultant product.
However, this technique requires the use of elevated temperature (at least 75.degree. C.) to dissolve the wax in the solvent and then cooling the solution to precipitate the wax. The wax does not stay in solution of ethyl acetate at ambient temperature (less than 50.degree. C.) which makes it very difficult to scale up production when using this methodology.
The shape of the toner particles has a bearing upon the electrostatic toner transfer and cleaning properties. Thus, for example, the transfer and cleaning efficiency of toner particles have been found to improve as the sphericity of the particles are reduced. Thus far, workers in the art have long sought to modify the shape of the evaporative limited coalescence type toners independently of pigment, binder, or charge agent choice in order to enhance the cleaning and transfer properties of the toner.
No prior art was found which suggested using SOLSPERSE.RTM. 24000 or 20000 dispersant in the manner of the present invention. The following documents disclose various toner preparations in which the use of a SOLSPERSE.RTM. 24000 dispersant is either claimed or disclosed: U.S. Pat. No. 5,108,863 to Hsieh et al.; U.S. Pat. No. 5,629,367 to Lofftus et al.; U.S. Pat. No. 5,399,454 to Imai et al.; U.S. Pat. No. 5,510,219 to Agata et al. and U.S. Pat. Nos. 5,298,355 and 5,298,356 to Tyagi et al.
U.S. Pat. No. 5,629,367 to Lofftus et al. describes a means to produce a dry pigment concentrate from a wet milled pigment utilizing SOLSPERSE.RTM. 24000, and subsequently, making electrophotographic toners by the conventional process of extrusion and classification of the toner. Limitations to the above conventional methodology include difficulty in attaining desired small particle size as well as narrow particle size distribution through a grinding process. Also, Lofftus teaches morphology and narrow particle size distribution of the dispersed pigment rather than controlled shape (sphericity) of the toner particles.